Vertical profiles of pelagic communities in the vicinity of the Azores Front and their implications to deep ocean ecology

Abstract

Three early wintertime and five early summertime stations were occupied in the vicinity of the Azores Front to the south-west of the islands. At each station the surface 1100–1400m of the water column were systematically sampled both by day and by night, using the Institute of Oceanographic Sciences' RMTI+8M system. The biological sampling was set in the context of an extensive study of the physical oceanography of the frontal region. It included one station in the core of an eddy which was seen to spawn from a meander during the course of the physical observations. The biological communities on either side of the front were almost indistinguishable in terms of their specific composition. However, there were substantial differences in the sizes of the standing-crops, the vertical distributions of the major taxonomic groups and the ranges of the diel vertical migrations being carried out by these groups. The waters on both sides of the front were oligotrophic. However, both phytoplankton standing-crop, as indicated by chlorophyll concentrations, and measures of productivity suggested that the water to the north (Eastern Atlantic Water), which was slightly cooler and with a shallower and steeper nutricline, was a little more productive than the water to the south (Western Atlantic Water). Associated with this trend in phytoplankton crop and productivity were quite sharp declines in the standing-crops of both plankton and micronekton across the front, moving from EAW to WAW. There was no evidence of enhanced productivity within the front itself, but there was an increase in the micronektonic standing-crop. Between the stations, there were consistent trends in the ratios of the standing-crops of plankton and micronekton, which were sampled simultaneously. By day, the planktonic biomasses exceeded the micronektonic biomasses by up to an order of magnitude in the wind-mixed layer. The ratios declined with depth, approaching unity at 200m during the wintertime, but remaining quite high to depths of 800m during the summer. At night, the ratios tended to be lower as a result of diel vertical migration, but except in one or two of the deepest samples the planktonic biomass always exceeded that of the micronekton. The total biomass profiles were very similar in shape between the stations, although there were quite large differences in the depths of the quartiles. There were substantial differences between the day and the night profiles. These could be attributed mostly to the effects of diel vertical migrations, but must also have included unquantifiable effects as a result of variations in net avoidance. The subdivision of the catches into the major taxonomic groups showed that the degree of avoidance varied substantially between them; euphausiids were the most effective daytime avoiders, but decapod crustaceans and fish showed a suprisingly small tendency to avoid. The ranges and the numbers of each group undertaking vertical migrations varied markedly between the stations, and these differences were expressed in the biomass estimates. Attempts have been made to quantify the movement of biomass in and out of the surface 200m and 500m resulting from these migrations. There were substantial changes in migratory behaviour across the front, even though there were no detectable changes in the light profiles. The data also imply that the micronektonic migrants play an important role in the active transport of organic substances down from the euphotic zone to mesopelagic depths (>500m), either in their gut contents or directly as a result of predation at depth. In addition, migrating plankton seem likely to be important in redistributing nutrients about the nutricline in these oligotrophic regions where the water column stability limits physical vertical mixing processes. Preliminary estimates suggest that their influence may be sufficiently large to alter the ratio between “new” and “old” production.